Binary (or bi-level) printing technologies such as wire matrix, thermal transfer, electrophotography and ink jet, place a mark or dot of constant optical intensity on the printing medium (e.g., paper) on demand. No dot is placed on the printing medium when printing is not required. Image and text are printed by proper placement of dots in pre-constructed patterns. Because of their binary nature, It is not a straightforward process to generate grayscales in these binary printing technologies.
Conventionally, a grayscale is generated by creating individual tone cells. A tone cell is defined as a matrix of juxtaposed dots. By placing various numbers of dots in certain dot positions of the matrix, while leaving the remaining dot positions blank, a grayscale composed of a number of gray tones can be generated. The practical resolution of the grayscale or tone images is reduced by the size (that is, the total number of dot positions) of the tone cells. For example, a 300 dots per inch (dpi) printing resolution with 3.times.3 tone cells results in a practical tone resolution of only 100 cells per inch. This produces resolution which is coarse enough to appear rather grainy to the human eye.
According to a particular example of prior art, W. Lama et al., "Hybrid (Gray Pixel) Halftone Printing," J. Imaging Tech., Vol. 15, No. 3, pp. 130-135, June 1989, incorporated herein by reference, the number of gray steps that can be achieved with the above binary tone cells is p+1, where p is the total number of dot positions in each tone cell. For example, a 3.times.3 tone cell would have 9 dot positions and 10 gray levels. See, e.g., FIG. 1A. In addition, the correlation between the measured optical densities and the assigned gray levels deviate substantially from linearity, signifying an unevenly distributed grayscale. See FIG. 1B. Furthermore, to print high-quality gray images, a large number of tone steps, e.g., greater than 64, is required. This makes it virtually impossible to achieve a large number of tone steps by using the above simple tone cell method in conjunction with a moderately low dot-resolution binary printer.
To increase the tone capability without forcing print engine design to a much higher resolution requiring elaborate and complicated hardware, there exist methods that would generate a large number of gray tones by introducing another dimension to the tone cells. One possible additional dimension is an intensity level for the individual dot. The default number of levels per dot is 2. That is, a dot matrix position can be either "white" (no dot) or "black" (with dot). An additional dimension may be added by increasing the number of intensity levels. Typically, this is achieved by the addition of several optical intensity (OI) levels at each dot position.
The above-cited reference by Lama et al. also gives formulas for calculating tone steps. For example, given a 3.times.3 tone cell, a 55 tone-step grayscale can be generated by the use of 3 levels of OI (including "white"), a 220 tone-step grayscale by 4 levels of OI, and a 715 tone-step grayscale by 5 OI levels.
To utilize this enhanced grayscale approach in binary printing, specific software and interface electronics have conventionally been required to translate the input image files to printing files containing the grayscale tone-cell transformation. Since different binary printing technologies can generate the required OI levels in a variety of ways, there were no convenient or standard architectures that were universally adopted or followed in the trade.
According to M. Takahashi et al., "Full-Color Ink Jet Printer Using Multilevel Ink," Soc. Inform. Display International Symposium Digest, Vol. 16, Orlando, Fla., Apr. 30-May 2, 1985, incorporated herein by reference, a print engine having either multiple printheads or a single printhead having multiple groups of active printing elements can be tailored to grayscale printing applications. For example, each of the printheads in the print engine or each group of active printing elements in the single printhead can be designed to produce dots of a specific optical intensity, either by producing specific spot sizes on the printing medium, usually paper, or by laying down marking substance of specific concentrations (e.g., by varying the amount of dyes in the ink) but keeping the dot size constant. For such architectures, the number of total OI levels is 1 plus the number of printheads or groups of active printing elements. The one extra OI level is attributable to the no-dot situation, i.e., the "white" background.
As a particular example of the prior art usage of multiple optical intensities, consider Sasaki, "Intermediate gradient image forming method," U.S. Pat. No. 4,714,964, issued Dec. 22, 1987, which discloses an image forming technique using two or more dot sizes and different coloring concentrations to form tone matrices. To suppress pseudo outlines which might take place when tone-cell optical density changes abruptly from one cell to the next, the optical density of the smallest dot comprising a high-concentration ink is set to be smaller than the optical density of the largest dot comprising a low-concentration ink.
As another example of the prior art, consider Hirahara et al., "Color image printing apparatus," U.S. Pat. No. 4,884,080, issued Nov. 28, 1989, which describes a color thermal dye transfer printer that prints a dot of a predetermined size corresponding to the density of each pixel of each color in a dot matrix corresponding to one pixel. The printer can produce a stable hue even if the positions of the printed dots of the respective colors are misregistered by a mechanical error and can prevent degradation in the image quality of the printed image due to moire fringes.
As still another example of the prior art, consider Allen, "Multitone ink jet printer and method of operation," U.S. Pat. No. 4,746,935, issued May 24, 1988, which discloses a method and apparatus useful for eight level halftone thermal ink jet printer by printing with ink droplets having volumes weighted in a binary sequence (that is, the droplets have volumes weighted by factors of 1, 2, and 4).
Three binary-weighted printing elements (drop generators) are sequentially fired at a chosen pixel, as the printing elements come into alignment with the pixel through the relative movement between the printhead and the paper. Firing one to three binary-weighted drop generators produces 1 to 7 volume units of ink within the pixel, thus producing an 8-level grayscale.
Alternatively, the optical density of ink ejected into a given pixel area during a halftoning printing operation may be reduced by ejecting a drop of untoned ink vehicle into the pixel before one or more ink droplets are ejected into the same area. This process eliminates the objectionable optical characteristics such as graininess, which are caused by individual high-contrast dots which are far enough apart, for a desired light gray tone, to be perceived as standing alone.
In the multi-element printhead of Allen, each of the three binary-weighted drop generators comprises a thin-film resistor and an orifice. The resistors and orifices vary in size so as to create the binary weighting effect. However, the Allen patent does not discuss designs for the heaters and the orifices such that multiple drops can address the same pixel as the printhead and the paper move, relative to each other.
As a further example of the prior art, consider Baker et al., "Thermal ink jet pen body construction having improved ink storage and feed capability," U.S. Pat. No. 4,771,295, issued Sep. 13, 1988, which describes multiple ink storage compartments communicating with a multi-orifice thermal ink jet printhead. These compartments preferably comprise reticulated polyurethane foams of controlled porosity and capillarity and are especially suitable for storing different inks, e.g., cyan, magenta and yellow color inks. The Baker patent is herein incorporated by reference.
Thus, several prior art systems described above have, in common, the drawback that they have special purpose architectures for the particular problems they seek to solve, and for the particular tasks they are customized for. It would be preferable to provide a more general purpose architecture, usable for a wider range of printing applications.